1. Field of the Invention
This invention relates generally to carbon/ceramic matrix composites and the method of making same. More particularly, the invention concerns carbon/ceramic matrix composites for use in friction applications.
2. Description of the Prior Art
With the rapid advance of aircraft, nuclear and aerospace and high temperature technologies there is an ever-increasing need for new types of high strength composite materials that are capable of reliably withstanding high temperatures and pressures. Additionally, new methods are required to expeditiously fabricate these materials into articles such as aircraft brake discs.
Carbon/carbon composites are the state-of-the-art friction materials for aircraft brake applications. The majority of today's commercial jets and military fighters are equipped with carbon/carbon brakes.
Carbon fibers, by design, have well oriented crystalline structure aligned along the axis of the fiber. They exhibit good strength and stiffness in the fiber direction.
The ability to combine different carbon fibers with different types of carbon matrices to form a single light weight, economical and functional composite is one of the key reasons for the successful application of carbon/carbon for aircraft brake use. The type and distribution of carbon fiber, the crystalline structures of various carbon matrices, the ratio between soft and hard carbon in the matrix, and the overall composite thermal conductivity all have an impact on final brake performance.
Carbon/carbon was first proposed as an aircraft frictional material in the beginning of 1970s. By the end of 1970s, carbon/carbon brakes were the standard equipment for advanced fighters such as F-14, F-15 and the supersonic Concorde.
Generally speaking, carbon/carbon brakes offer low wear and provide excellent frictional performance at high energy conditions. Additionally, the use of carbon/carbon aircraft brakes significantly adds safety and increases payload.
Prior art carbon/carbon aircraft brakes are generally composed of multiple full-circle rotors and stators of the same material.
The unique friction properties of structural carbon/carbon composite brake materials have now been fully established for multi-disc rotor/stator braking systems for commercial and fighter aircraft, as well as for caliper/single disc applications for helicopters, industrial, automotive and train braking applications. Carbon/carbon braking materials are currently manufactured in large volume production quantities, especially for commercial and military aircraft. Wear life and friction coefficients are at predictable levels and cannot be significantly influenced by carbon/carbon processing conditions.
The technology for densifying carbon fiber substrates by liquid pitch or resin impregnation, carbonization and graphitization or chemical vapor infiltration of pyrolytic carbon, with subsequent composite heat treatment is fully established and a variety of carbon/carbon products are routinely manufactured including complex aerospace components, high temperature furnace hardware, components for the Semi-Conductor Industry, brake discs for commercial and military aircraft, as well as for automotive and other commercial applications.
Densification by the chemical vapor infiltration (CVI) process is the most popular manufacturing process in the industry to date for the manufacture of carbon/carbon composite friction materials for aircraft braking systems. Fiber volume for the carbon substrates may range from 20%-30%. Depending on initial carbon fiber density, fiber volume and number and length of pyrolytic carbon infiltration furnace runs, the fully densified carbon/carbon composite product may range in density from 1.5 g/cc to 1.85 g/cc.
Two of the early patents concerned with carbon/carbon aircraft brakes and the methods for making the brake discs, namely U.S. Pat. Nos. 3,895,084 and 3,991,248 were issued to the present inventor. These patents describe unique substrate optimization techniques as well as novel methods for accurate control of product shape, cross-sectional configuration, density, fiber volume and internal fiber orientation.
One of the drawbacks of prior art carbon/carbon brakes is that they typically yield lower frictional coefficients, which tend to vary widely at different speed and landing energy. Carbon/carbon is also susceptible to oxidation damage, which not only degrades its structural integrity over long-term usage, but also promotes accelerated wear.
In the past, considerable development work has been carried out to develop a ceramic matrix composite (CMC) friction material that exhibits improved friction properties over carbon/carbon brake materials.
These efforts have largely focused on material systems that are based on either silicon melt infiltration and carbide conversion, or pre-ceramic polymer impregnation of carbon fiber mats, resulting in ceramic matrix composites after pyrolyzation. This work has to date, not been particularly successful for aircraft braking applications.
Another early development effort is described in U.S. Pat. No. 5,153,295 issued to whitmarsh et al., entitled “Carbosilane Polymer Precursors To Silicon Carbide Ceramics”. This patent describes a process for the preparation of compositions of matter which have potential utility as precursors to silicon carbide (SiC) wherein the compositions are obtained by a Grignard coupling process starting from chlorocarbosilanes. The precursors constitute a type of polycarbosilane that is characterized by a branched, [Si—C].sub.n“backbone” comprised of SiR.sub.3 CH.sub.2--, --SiR.sub.2 CH.sub.2--, .dbd.SiRCH.sub.2--, and .tbd.SiCH.sub.2-- units (where R is usually H but can also be other organic or inorganic groups, e.g., lower alkyl or alkenyl, as may be needed to promote crosslinking or to modify the physical properties of the polymer or the composition of the final ceramic product). A key feature of these polymers is that substantially all of the linkages between the Si—C units are “head-to-tail”, i.e., they are Si to C.
Recently, considerable effort has been directed toward developing ceramic matrix composites (CMC) that are specially aimed at aircraft braking applications. Much of this work has been based on pre-ceramic polymer impregnation of carbon fiber preforms that may undergo as many as twelve polymer impregnations before the desired final density is reached. Typically the pre-ceramic polymer forms the ceramic matrix after pyrolyzation between about 850° C. and about 1600° C.
One of the goals of the present invention is to further optimize carbon/ceramic friction material by identifying key material process variables and systematically correlating the resulting carbon/ceramic matrix composites with brake performance and to develop reproducible and cost effective processing steps to fabricate carbon/ceramic matrix composites for future brake applications.